In general, the separators used in batteries (whether rechargeable or not) need to have the following properties:
high electrical resistivity in order to avoid short circuit currents flowing between the electrodes;
sufficient porosity to allow ions to pass between the cathode and the anode compartments;
high mechanical strength in order to withstand handling stresses during manufacture, and the stresses due to changes in electrode volume during charging and discharging processes;
the ability to withstand chemical attack by the highly concentrated alkaline electrolyte, by impurities, and by the electrode materials; and
low cost and ease of manufacture.
The separators conventionally used in alkaline and in saline primary cells or "batteries" are constituted by woven or non-woven, vinyl, cellulose, or olefin fibers. Their qualities then result from an appropriate adjustment between porosity, mechanical strength, wettability by liquids, and thickness.
The porosity of a separator is adjusted to prevent particles from either electrode going through the separator and reacting with the other electrode which would give rise to self-discharge possibly accompanied by the evolution of gas inside the cell. One way of achieving this result is to increase thickness or to increase the number of layers in the separator, however this entrains the drawback of reducing the volume available for the anode and the cathode materials.
In addition, specific problems arise for each type of electrochemical cell.
Thus, in rechargeable cells including a negative electrode based on zinc and an alkaline electrolyte based on concentrated sodium or potassium hydroxide, it is well known that the electrochemical cycling of the zinc electrode in successive charges and discharges gives rise to major drawbacks. Thus, there is the problem of the zinc electrode changing shape due to preferential redeposition of zinc on the central or bottom face of the current collector. This phenomenon increases the thickness of the electrode at certain points and gives rise to higher and higher current densities, thereby reducing the efficiency of the zinc at charging electricity. In addition, mechanical stresses due to the increasing thickness and to the change in shape of the electrode are generally fatal for the lifetime of a storage cell.
A second problem that arises with such storage cells during charging is the appearance and growth of zinc dendrites on the surface of the electrode. During successive cycles, dendrite growth can continue until the space between the zinc negative electrode and the positive electrode is bridged, thereby setting up an internal short circuit. One known way of avoiding this problem is to use separators characterized by good mechanical resistance to thrust from dendrites or by low permeability to diffusion of zincate ions, so as to limit concentration gradients in the electrolyte, since such gradients encourage the dendrite growth.
For metal-air type primary cells, having some or all of the positive electrode constituted by a porous compacted mass based on a metal oxide such as manganese dioxide (MnO.sub.2), and regenerable by oxygen flowing through the pores of the electrode, another major problem occurs which is totally different from that which arises in zinc electrode storage batteries.
With a positive electrode constituted by a mass which is porous to air, there exists some flow of oxygen within the electrode right up to the immediate vicinity of the separator. More precisely, oxygen in the air penetrates into the cell via the pores of the positive electrode and is then reduced to water at active sites (catalysts) on the air electrode if present or on the manganese dioxide of the positive active mass. However, some of the oxygen may avoid reaction and diffuse through the separator towards the negative electrode. The material from which the negative electrode is constituted then oxidizes chemically. This self-discharge mechanism reduces the capacity of the electrochemical cell. Such self-discharge is particularly problematic under conditions of intermittent utilization where the service life of the cell may be several months.
Similarly, during storage, if the cell is not protected by gas-tight packaging, then the metal constituting the negative electrode will oxidize and give rise to a major loss in capacity.
For example, a zinc based anode gel as currently used in alkaline batteries may loose between 20% to 80% of its capacity over a period of one year.
Several patents describe separators which use polyvinyl alcohol and which have the property of limiting the diffusion of ions in the electrolyte (in particular zincates) in order to avoid zinc dendrite growth or perforation of the separator in silver-zinc, nickel-zinc, or air-zinc rechargeable storage cells.
However, these separators give poor results in metal-air type primary cells. They are characterized by major cross-linking of the polyvinyl alcohol in order to ensure good mechanical strength for resisting piercing by dendrites, and in order to establish a lattice of macromolecular chains constituting a barrier to the passage of zincate ions in solution in the electrolyte.
This high degree of cross-linking reduces the hydrophilic properties of the separator and consequently reduces its ability to absorb highly concentrated alkaline electrolyte.
Another drawback of such separators is their relatively high electrolytic resistance which limits the power that the cell can deliver in high power consumer market applications (radios, flash guns, tape recorders, etc.).
U.S. Pat. No. 4,361,632 describes a composite separator in which cross-linking is performed hot, enabling ion diffusion to be limited, and in particular limiting the diffusion of zincates in rechargeable zinc electrode storage batteries.
French patent No. FR-A-2 251 922 describes a separator having a plurality of layers comprising a porous layer and a semipermeable membrane constituted by a derivative of polyvinyl alcohol. The method described seeks to avoid having the porous layer embedded at depth, and to limit impregnation in a surface zone of the porous layer. The resulting highly cross-linked PVA film is doubtless effective at limiting the diffusion of zincates in storage batteries having rechargeable zinc electrodes, but it does not limit the diffusion of very small molecules such as molecules of oxygen in gaseous form or dissolved in the electrolyte.
French patent No. FR-A-2 110 486 describes an alkaline storage battery including a sheet of highly cross-linked polyvinyl alcohol surrounding the zinc electrode and retaining the zincates.
U.S. Pat. No. 3,892,592 and Japanese patent No. JP-A-7 567 428 describe the use of polyvinyl alcohol in C/Zn saline batteries, as a separator paste with a saline electrolyte and a support made of paper in order to ensure good reactivity for the solid zinc electrode in the form of a container.
The object of the present invention is to solve problems specific to primary metal/air type alkaline electrochemical cells, and to find a separator which is satisfactory for use in such cells.